1. Field of the Invention
The present invention relates to a manufacturing method for a sealing plug used in sealing an arc tube, the sealing plug, and a discharge lamp that uses the sealing plug.
2. Related Art
In discharge lamps, and in particular high-pressure mercury lamps, the electrodes and the external lead wire for supplying power to the electrodes are generally connected to one another via a metallic foil mounted within the arc tube. This is to prevent the light-emitting material filled within the arc tube from leaking out under conditions of high-temperature and high-pressure.
This structure is satisfactory when the discharge lamp is not frequently operated. However, when the frequency with which the discharge lamp is operated increases, cracking may occur due to thermal stress resulting from a difference in the coefficients of thermal expansion of the metallic foil and the arc tube. The light-emitting material within the arc tube then leaks out as a result of the cracking, and the life of the lamp is thus shortened.
In order to resolve this problem, a method of sealing the ends of the arc tube using a sealing plug has been suggested (see application no. 5-290810 filed in Japan).
This sealing plug is obtained by sintering a preform formed from a plurality of layers positioned concentrically around a metal lead wire that supplies power to the electrodes within the arc tube of a high-pressure mercury lamp. Each layer, in a direction from the inner to the outer layers, includes less of the metal lead wire component and more of the arc tube component. As such, the coefficient of thermal expansion of the sealing plug gradually changes, from the inner layers to the outer layers, from having a value close to the coefficient of thermal expansion of the metal lead wire to having a value close to the coefficient of thermal expansion of the arc tube. Consequently, even if the temperature within the arc tube increases, the thermal stress generated between the metal lead wire and the arc tube is gradually relieved in the intermediate layers of the sealing plug, and thus the cracking described above can be prevented.
Manufacturing methods for such a sealing plug are disclosed in the application cited above. One method includes forming the layers of a preform by repeating the process of applying slurries (made from the powders of components of the metal lead wire and the arc tube, as well as organic binders, organic solvents, dispersants, and the like) to the metal lead wire and drying the applied slurries, and then sintering the preform thus formed. Another method includes manufacturing green sheets corresponding to the layers formed from the slurries, wrapping the green sheets around the metal lead wire to form a preform, and sintering the preform thus formed.
Although the above methods for manufacturing the sealing plug are effective in the prevention of cracking, there are problems relating to the quality and manufacturability of the sealing plug.
Specifically, in the method according to which layers are formed by applying slurries to the metal lead wire and drying the applied slurries, problems relating to the size and shape of the sealing plug (e.g. variations in the amount of slurry applied; variations in the thickness of individual layers), and the layers not been concentrically positioned with respect to the metal lead wire, can occur, and as a result it is difficult to manufacture sealing plugs of an even quality.
On the other hand, in the method according to which layers of green sheets are formed around the metal lead wire, it is extremely difficult to wrap the green sheets evenly around the metal lead wire, which has a small diameter, and to layer the green sheets so that the ends meet exactly (i.e. without the ends either overlapping or not meeting). Thus this method is of little practical use.